How can i divide like triangle?
The tool you are looking for is called 'Poke Faces'.

If can also be activated from the Mesh menu in edit mode under the Faces submenu (i.e.  Mesh ‣ Faces ‣ Poke Faces). 

see https://docs.blender.org/manual/en/latest/modeling/meshes/editing/faces.html#poke-faces for more info.

In Blender 2.8, the Faces sub menu has been moved out to be its own full menu. It also appears that the alt-p shortcut is not defined, so that is likely to be your problem.
See: